1. Field of the Invention
The subject of the present invention is a device for treatment of blood by extracorporeal circulation, designed to produce a sterile and pyrogen free treatment liquid.
2. Description of the Related Art
Patients suffering from renal insufficiency may undergo various treatments involving extracorporeal circulation of blood, haemodialysis, haemofiltration and haemodiafiltration.
Haemodialysis consists in circulating the blood of the patient in a first compartment of a dialyser, and a dialysis liquid in a second compartment of the dialyser, the two compartments being separated by a semipermeable membrane allowing diffusive transfer of solutes through the membrane, from the compartment in which the concentration of a particular solute is the highest to the compartment in which the concentration of this solute is the lowest.
Haemofiltration consists in extracting a fraction of plasmatic water or filtrate from the blood of the patient by means of a haemofilter and in simultaneously infusing into the patient a substitution liquid in order to compensate, in general only partially, for the quantity of filtrate extracted.
Haemodiafiltration is a combination of the two aforementioned treatments.
The dialysis liquid and the substitution liquid are liquids which have substantially the same composition. They are isotonic with the blood of which they contain the main electrolytes.
In conventional dialysis machines, the dialysis liquid is prepared by mixing water and concentrated solutions or pulverulent salts comprising the main electrolytes of blood. This dialysis liquid is neither sterile nor pyrogen free, that is to say that it may contain living microorganisms (bacteria) as well as components called pyrogens, the introduction of which into the body may produce undesirable effects, such as fever, shivering, nausea or anaphylactoid reactions.
Although the membrane of the dialyser isolates the blood from the dialysis liquid and, during treatment, a positive transmembrane pressure is set up between the blood compartment and the dialysis-liquid compartment of the dialyser in order to prevent the passage of dialysis liquid into the blood, not all contamination of the blood by bacteria and pyrogenic components contained in the dialysis liquid is completely excluded, especially in the event of breakage of the filter or in the event of accidental reversal of the direction of the transmembrane pressure when a dialyser with high hydraulic permeability is used.
Repeated interest in the use of sterile and pyrogen free dialysis liquid has moreover been expressed. It has, in particular, been proposed to filter the dialysis liquid in a filter having a first chamber and a second chamber which are separated by a membrane having a high hydraulic permeability, the first chamber having an inlet for introducing the liquid to be filtered and an outlet for discharging the substances trapped by the membrane together with a fraction of the liquid introduced into the filter (see "Investigation of the Permeability of Highly Permeable Polysulfone Membranes for Pyrogens" in Contr. Nephrol., vol. 46, pp. 174-183, Karger, Basel 1985).
European Patent 0,270,794 describes a dialysis machine whose dialysis liquid circuit comprises:
a supply tube on which is arranged a filter having a first chamber and a second chamber which are separated by a filtering membrane, the supply tube having a first portion connecting a dialysis liquid source to an inlet of the first chamber, and a second portion having an end connected to an outlet of the second chamber of the filter and another end which can be connected to an inlet of a first compartment of a dialyser; PA1 a discharge tube having an end which can be connected to an outlet of the first compartment of the dialyser; and PA1 a purge tube on which a valve is arranged connecting an outlet of the first chamber of the filter to the discharge tube. PA1 a supply tube on which is arranged a filter having a first chamber and a second chamber which are separated by a filtering membrane, the supply tube having a first portion connecting a dialysis liquid source to an inlet of the first chamber, and a second portion having an end connected to an outlet of the second chamber of the filter and another end which can be connected to an inlet of a compartment of an exchanger with semipermeable membrane; PA1 a discharge tube having an end which can be connected to an outlet of the compartment of the exchanger; and PA1 a purge tube on which a flow-control element (for example a valve) is arranged and which is connected to an outlet of the first chamber of the filter, PA1 characterized in that it comprises a feedback tube connecting the inlet of the first chamber of the filter to the outlet of the first chamber, and on which is arranged a flushing pump for circulating liquid in the first chamber of the filter and causing cleaning of the membrane by tangential flushing. PA1 measuring a clogging level of the filter; PA1 comparing the clogging level measured with a predetermined clogging threshold; and PA1 purging the first chamber of the filter when the clogging level measured reaches the predetermined clogging level. PA1 measuring and storing in memory, for a given treatment liquid flow rate, a reference transmembrane pressure in the filter the first time the filter is set in operation; PA1 measuring, on each subsequent use of the filter, the transmembrane pressure at the given treatment liquid flow rate; PA1 comparing the transmembrane pressure measured with the reference transmembrane pressure.
During operation, the valve arranged on the purge tube is opened at regular time intervals in order to purge the first chamber of the filter of the microorganisms and pyrogenic components trapped by the membrane.
This machine has several drawbacks. In particular, it is clear that the valve of the purge tube cannot be opened frequently because of the interruption to the treatment which results therefrom. (As a matter of fact, because of the significant head loss which the membrane of the filter constitutes, when the valve is opened, any dialysis liquid coming from the dialysis liquid source flows through the purge tube and the dialyser is no longer supplied with fresh dialysis fluid). Furthermore, between two successive openings of the valve, there is a time lag during which the undesirable substances accumulate in the first chamber of the filter where they tend to be entrained by convection towards the membrane and to clog it. The result of this is, in the event of breakage of the membrane of the filter, that these accumulated substances are sent into the dialyser and the dialysis circuit portion located upstream and downstream thereof, which these substances contaminate. Also, the tangential flushing of the membrane which results from intermittent opening of the valve cannot be sufficiently long to detach from the membrane all the substances which have adhered thereto.